1. Field of the Invention
The present invention relates to a control apparatus for an engine, and more particularly, to a control apparatus for a direct-injection, spark-ignition engine equipped with a catalyst in its exhaust-gas passage, a tumble generating section, and a fuel injector for directly injecting fuel into a combustion chamber so as to stratify combustible mixture in the proximity of a spark plug by way of the tumble flow during a stratified-combustion operation.
2. Description of the Related Art
Conventionally, direct-injection, spark-ignition engines have been well known. Typically, such engines are provided with a fuel injector which directly injects fuel into a combustion chamber, wherein an air/fuel ratio is set to be lean and the fuel injector injects fuel during a compression stroke so as to concentrate mixture in the proximity of the spark plug for providing the stratified-combustion operation in an operational region of low engine rotational speed and low load, thereby improving fuel efficiency.
In the meantime, generally, an automotive engine is equipped with a catalyst in its exhaust-gas passage for purifying exhaust-gas. Because the catalyst provides insufficient purification in an inactivated state where its temperature is lower than its activation temperature, the catalyst desirably promotes its warm-up (referred to as light-off) to activate as quickly as possible for better emission performance.
By way of example of an approach to promoting light-off of the catalyst for the direct-injection spark-ignition engine described above, Japanese Patent Publication No. 2000-54881 discloses controlling the fuel injector to perform a divided fuel injection of a plurality of fuel injections during the inactivated state of the catalyst. That is, in the engine disclosed in the patent publication above, the divided injection performed during a compression stroke in the inactivated state of the catalyst produces a mixture layer richer in an air/fuel ratio in the proximity of the spark plug and a mixture layer leaner in an air/fuel ratio around the richer layer at an ignition timing. Ignition under such mixture distribution raises the exhaust-gas temperature to promote light-off of the catalyst, as will be described later.
Meanwhile, an embodiment illustrated in drawing figures of the patent publication above discloses a cavity constituted of a deep recess formed at a portion of top surface approximate to the fuel injector, for guiding fuel injected by the fuel injector. Fuel is injected towards the cavity, so that fuel spray collides against inner surfaces of the cavity and is directed towards the spark plug.
Some types of the aforementioned engines perform a divided injection of an intake-stroke injection and a compression-stroke injection in the inactivated state of the catalyst. In these engines, as well as the one described above, the compression-stroke injection produces a richer mixture-layer in the proximity of the spark plug and a leaner mixture-layer outside of the richer layer (or the area up to a peripheral wall of the combustion chamber), so as to raise the exhaust-gas temperature for promoting light-off of the catalyst.
Moreover, Japanese Patent Publication No. H11-141338 proposes an approach to effectively stratifying mixture in the proximity of the spark plug, other than guiding the fuel spray by way of the wall surface of the cavity described above in the direct-injection spark-ignition engine. The approach is to configure an intake port so as to generate a tumble flow in the combustion chamber, and to cause the fuel injector to inject fuel in the opposite direction to the tumble flow during the late-stage of the compression stroke in the stratified-combustion mode, so that the fuel sprayed by the fuel injector is redirected by the tumble flow and transferred to the proximity of the spark plug.
The divided injections during the compression stroke or the divided injections during the intake stroke and the compression stroke produce a relatively richer mixture-layer in the proximity of the spark plug and a relatively leaner mixture-layer around the richer mixture as described above. This can prolong the combustion because the relative richer mixture-layer in the proximity of the spark plug combusts immediately after the ignition, the flame is propagated to the leaner mixture-layer thereafter, and then the leaner mixture-layer slowly combusts. Additionally, excessive air and the remaining fuel are mixed in the proximity of the spark plug to further combust. As a result, the combustion continues until a late timing.
The delayed combustion-completion raises the exhaust-gas temperature. However, for an additional temperature increase in the exhaust-gas to promote light-off of the catalyst, the ignition timing is desirably retarded in connection with the retard of the fuel injection timing while maintaining the sufficient combustibility to prevent the deterioration in emission performance and fuel efficiency. Japanese Patent Publication No. H11-141338 refers to no particular control in the inactivated state of the catalyst.
In view of the aspect above, an object of the present invention is to provide a control apparatus for a direct-injection, spark-ignition engine, which further promotes light-off of the catalyst than the conventional apparatus while avoiding the deterioration in emission performance and fuel efficiency in the inactivated state of the catalyst.
In accordance with the present invention, the object is achieved by providing a control apparatus for a direct-injection, spark-ignition engine including a tumble generating device for generating a tumble flow in a combustion chamber, a fuel injector which injects fuel in the direction opposite to the tumble flow into the combustion chamber so as to generate a combustible mixture adjacent to a spark plug at an ignition timing in association with the ignition timing in a predetermined operational region of low engine rotational-speed and low load, and a catalyst disposed in an exhaust-gas passage, the control apparatus including a temperature-condition detection device for detecting a temperature condition of the catalyst, and a control device for controlling a fuel-injection operation of the fuel injector. In accordance with the present invention, the control device controls the fuel injector to perform divided fuel injections of at least a leading fuel injection and a trailing fuel injection during a compression stroke prior to an ignition timing based upon the determination of the temperature condition by the temperature-condition detection device in an inactivated state of the catalyst. Moreover, the trailing fuel injection is performed at such a timing that its mixture concentrates in the proximity of the spark plug at the ignition timing by way of high cylinder pressure during the late-stage of the compression stroke, the leading fuel injection is performed at such a timing that its mixture distributes around the mixture layer by the trailing fuel injection at the ignition timing, and the midpoint between an initiation timing of the leading fuel injection and an initiation timing of the trailing fuel injection is positioned within a period during the latter half of the compression stroke.
Accordingly, at the ignition timing, a mixture distribution, in which relatively richer mixture-layer of fuel spray by the trailing fuel injection concentrates in the proximity of the spark plug and a relatively leaner mixture-layer of fuel spray by the leading fuel injection distributes around the richer layer, is attained. Under such a mixture distribution, the relative richer mixture-layer in the proximity of the spark plug combusts firstly, the flame is propagated to the leaner mixture-layer thereafter, and then the leaner mixture-layer slowly combusts. Subsequently, excessive air and the remaining fuel are mixed adjacent to the spark plug to combust. Therefore, an overall combustion period is prolonged, or the combustion completion timing is delayed, so as to effectively raise the exhaust-gas temperature. That is, in a direct-injection, spark-ignition engine that achieves the stratified mixture distribution by the confronting of a tumble flow and intake air, an adequate mixture distribution for promoting the temperature rise in the exhaust gas is reliably provided. Particularly, the trailing fuel injection is performed in the late-stage of the compression stroke during which the cylinder pressure is relatively higher, so that the fuel spray by the trailing injection remains adjacent to the spark plug. On the other hand, the leading fuel injection is retarded and performed during the term when the tumble flow is relatively stronger, so that the fuel spray by the leading fuel injection confronts against the tumble flow to be quickly atomized and distributed around the spark plug. As a result, at the ignition timing, the mixture of the fuel spray by the leading injection distributes over a certain area around the spark plug and a richer mixture-layer enriched by the fuel spray of the trailing injection concentrates in the proximity of the spark plug, in the direct-injection spark-ignition engine that guides the fuel spray with the tumble flow generated in the combustion chamber.
Additionally, the initiation timing of the leading fuel injection and the initiation timing of the trailing fuel injection are delayed so that the midpoint between the initiation timing of the leading fuel injection and the initiation timing of the trailing fuel injection is positioned within the latter half of the compression stroke. Especially, the trailing fuel injection timing is delayed to a period close to the top dead center of the compression stroke so that its fuel spray concentrates in the proximity of the spark plug at the ignition timing by way of high pressure during the late-stage of the compression stroke, and the ignition timing is retarded in connection with the delay of the injection timing. The retarded ignition timing and the delayed injection timing in the aforementioned manner effectively raise the exhaust-gas temperature.
Moreover, in accordance with the present invention, the fuel injector injects fuel in the direction opposite to the tumble flow, so that less fuel deposits on the wall surface causes preferable combustion, which provides less smoke production and less HC emission, even if the injection timing is delayed. This improves emission performance and fuel efficiency. In contrast, in accordance with the conventional apparatus provided with a deep cavity on the top surface of its piston for guiding fuel spray by way of the wall surface of the cavity, the amount of fuel deposit increases as the fuel injection timing is delayed, which is likely to deteriorate combustibility and increase smoke production.
Preferably, in the apparatus set forth in the present invention, the leading fuel injection may be performed at such a timing that its fuel spray confronts against the tumble flow to concentrate adjacent to the spark plug. This provides a mixture distribution in which a relatively leaner-mixture, which is fully atomized after colliding against the tumble flow, distributes around the relatively richer mixture-layer by fuel spray of the trailing fuel injection, ensuring the combustion stability.
More preferably, the amount of the leading fuel injection may be larger than that of the trailing fuel injection. That is, the trailing fuel injection, whose fuel spray is difficult to atomize, is in a relatively smaller amount, on the other hand, the leading fuel injection, whose fuel spray is easy to atomize, is in a relatively larger amount, which is advantageous to smoke reduction.
Further, it is preferred that the fuel injector be controlled so as to perform the leading fuel injection and the trailing fuel injection during the late-stage of the compression stroke prior to an ignition timing in the inactivated state of the catalyst, at least in a condition of low engine rotational-speed and low load. Still further, it is preferred that both the timings of the leading fuel injection and the trailing fuel injection be set so that the midpoint between a completion timing of the leading fuel injection and an initiation timing of the trailing fuel injection is positioned within a period past two thirds of the compression stroke.
According to the constitutions above, retarding timings of the leading fuel injection and delaying the trailing fuel injection as possible within the range where the preferable combustibility is maintained in the aforementioned manner further raises the exhaust-gas temperature in combination with the ignition timing greatly retarded.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment relative to the accompanied drawings.